The present invention relates to a laser beam printer and, more specifically, to a color laser beam printer which can produce printed images of a plurality of colors.
Conventionally, what is called a tandem-type color laser printer is known which is constituted of a plurality of scanning optical systems and a plurality of electrophotographic units as shown in FIG. 22. In FIG. 22, latent images of, for instance, cyan, magenta, yellow and black are written, by scanning optical systems 20a-20d, to photoreceptor drums 21a-21d that have been charged by means of charging devices 26a-26d such as corotrons, respectively. Toner images obtained by developing the latent images by respective developing devices 25a-25d are transferred onto each of sheets 22a-22c that are being transported on a transport belt 23, and then fused by a fusing device 24 to form into a color image. The photoreceptor drums 21a-21d are cleaned by respective cleaners 27a-27d.
A tandem-type color laser printer needs a plurality of scanning optical systems; for instance, the printer of FIG. 22 has the four scanning optical systems. Decreasing the number of scanning optical systems is expected to greatly reduce the cost. In particular, decreasing the number of polygon scanners will reduce not only the cost but also the power consumption.
For the above reason, several attempts have conventionally been made to decrease the number of polygon scanners. For example, Japanese Patent Application Unexamined Publication No. Sho. 58-23074 discloses a technique in which two polygon scanners are provided in a tandem-type color laser beam printer having four photoreceptors, and two photoreceptors are alternately exposed with a laser beam from one polygon scanner by a switching operation of a switching mirror.
However, in this apparatus, the mechanical optical path switching operation of the switching mirror makes the configuration of the apparatus complex, resulting in an increased cost and a difficulty in exposing the photoreceptors with a high positional accuracy. Further, the existence of movable portions makes the apparatus less reliable.
Japanese Patent Application Unexamined Publication No. Hei. 3-42612 discloses a technique in which one polygon scanner is provided in a tandem-type color laser beam printer having four photoreceptors, and four laser beams are input from both sides of the polygon scanner and reflected by different mirror faces to effect simultaneous scans by the four laser beams.
However, this apparatus uses only the single polygon scanner, the optical systems need to be provided independently for the respective laser beams. Therefore, the size of the whole optical system cannot be reduced sufficiently.